[Mesa-dev] [PATCH 1/2] nir: add lowering for ffloor

[Jason Ekstrand]

On Mon, Nov 12, 2018 at 1:29 PM Christian Gmeiner <
christian.gmeiner at gmail.com> wrote:

> Hi Jason
>
> Am Sa., 1. Sep. 2018 um 21:23 Uhr schrieb Jason Ekstrand <
> jason at jlekstrand.net>:
> >
> > I don't think either of these work for negative numbers
> >
>
> I would like to land this patch - can you provide some details why
> this does not work for
> negative numbers?
>

No, this is correct.  It's GLSL's definition of fract(x) that's dumb.  GLSL
defines fract(x) as x - floor(x) (exactly what your formula says.  This
means that

fract(-1.4) = -1.4 - floor(-1.4) = -1.4 - (-2.0) = -1.4 + 2.0 = 0.6

so GLSL fract() doesn't give you the fractional part at all.  *sigh*

Reviewed-by: Jason Ekstrand <jason at jlekstrand.net>


> > On September 1, 2018 14:16:11 Christian Gmeiner
> > <christian.gmeiner at gmail.com> wrote:
> >
> > > Signed-off-by: Christian Gmeiner <christian.gmeiner at gmail.com>
> > > ---
> > > src/compiler/nir/nir.h                | 3 +++
> > > src/compiler/nir/nir_opt_algebraic.py | 1 +
> > > 2 files changed, 4 insertions(+)
> > >
> > > diff --git a/src/compiler/nir/nir.h b/src/compiler/nir/nir.h
> > > index 169fa1fa20..d81eefc032 100644
> > > --- a/src/compiler/nir/nir.h
> > > +++ b/src/compiler/nir/nir.h
> > > @@ -2054,6 +2054,9 @@ typedef struct nir_shader_compiler_options {
> > >     */
> > >    bool fdot_replicates;
> > >
> > > +   /** lowers ffloor to fsub+ffract: */
> > > +   bool lower_ffloor;
> > > +
> > >    /** lowers ffract to fsub+ffloor: */
> > >    bool lower_ffract;
> > >
> > > diff --git a/src/compiler/nir/nir_opt_algebraic.py
> > > b/src/compiler/nir/nir_opt_algebraic.py
> > > index ae1261f874..3d2b861a42 100644
> > > --- a/src/compiler/nir/nir_opt_algebraic.py
> > > +++ b/src/compiler/nir/nir_opt_algebraic.py
> > > @@ -118,6 +118,7 @@ optimizations = [
> > >    (('~flrp', a, 0.0, c), ('fadd', ('fmul', ('fneg', a), c), a)),
> > >    (('flrp at 32', a, b, c), ('fadd', ('fmul', c, ('fsub', b, a)), a),
> > >    'options->lower_flrp32'),
> > >    (('flrp at 64', a, b, c), ('fadd', ('fmul', c, ('fsub', b, a)), a),
> > >    'options->lower_flrp64'),
> > > +   (('ffloor', a), ('fsub', a, ('ffract', a)),
> 'options->lower_ffloor'),
> > >    (('ffract', a), ('fsub', a, ('ffloor', a)),
> 'options->lower_ffract'),
> > >    (('~fadd', ('fmul', a, ('fadd', 1.0, ('fneg', ('b2f', c)))),
> ('fmul', b,
> > >    ('b2f', c))), ('bcsel', c, b, a), 'options->lower_flrp32'),
> > >    (('~fadd at 32', ('fmul', a, ('fadd', 1.0, ('fneg',         c ))),
> ('fmul', b,
> > >            c )), ('flrp', a, b, c), '!options->lower_flrp32'),
> > > --
> > > 2.17.1
> > >
> > > _______________________________________________
> > > mesa-dev mailing list
> > > mesa-dev at lists.freedesktop.org
> > > https://lists.freedesktop.org/mailman/listinfo/mesa-dev
> >
> >
> >
>
> --
> greets
> --
> Christian Gmeiner, MSc
>
> https://christian-gmeiner.info
>
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